Various approaches for the conversion of coal to oil, gas and other chemical by-products have been under development since the mid 1800's. Conversion approaches utilizing electricity have generally been limited to either an electro-chemical process or an electro-thermal gasification scheme.
The first work on electro-chemical coal conversion was reported in the late 1800's by Bartoli. This and later electro-chemical studies have emphasized the conversion in aqueous solution using acids and other catalysts. These catalysts promote oxidation and reduction reactions that lead to the conversion of the coal.
While the electro-chemical processes have been shown to successfully convert coal to oil, gas and related by-products, they, disadvantageously, proceed slowly. Further, up to this point in time, electro-chemical processes have only been utilized for above ground conversion and have not been adapted for utilization on coal in situ. As a consequence, conversion of coal by electro-chemical processes have not proved economically feasible. This is primarily due to the cost of first winning the coal and then transporting the coal to the conversion site and finally processing the coal for subsequent electro-chemical conversion.
The additional winning, transporting and processing expenses inherent in prior art electro-chemical conversion methods may be avoided if the coal is converted in situ. Such alternative underground conversion methods have been under study in the United States since about the 1950's.
To the best of this inventor's knowledge, the underground conversion techniques that have been heretofore developed relate primarily to electro-thermal gasification schemes. These schemes utilize electricity as a heat source.
One of the first successful approaches is disclosed in the article, "A Fresh Try at Underground Gasification" appearing in the April, 1951 issue of "Chemical Engineering". Stainless steel pipe electrodes are driven from the surface down into the coal seam. During the first stage of the two stage process, an electric current is passed between the electrodes and the coal is heated by its own electrical resistance. As the coal carbonizes, coal gases are released. These gases pass through perforations into the pipe electrodes and then up through the pipes where they are collected at the surface.
During the second stage of the process air, oxygen and/or steam is passed down through the pipe electrodes and through channels in the coal seam previously created during the first stage. The carbonized coal or coke is ignited and the producer gas that is formed is recovered through the electrode pipes as described above.
While coal is successfully converted utilizing this method, it suffers from a number of disadvantages that have prevented its effective commercialization. The electrical current passing between the electrodes follows the path of least resistance through the coal seam. Thus, the current follows an unpredictable path along moisture pockets and crevices in the coal. This results in random, uncontrolled tunnel formation. Often the resulting tunnel formation leads directly to undesirable patterns of subsidence.
Another technique of electro-thermal gasification designed to avoid this problem is disclosed in U.S. Pat. No. 4,067,390 to Camacho, et al. This patent relates to the application of a plasma arc underground in a coal seam to produce acetylene and other by-products.
The method utilizes a plasma torch including inner and outer concentrically disposed electrodes of opposite polarity. A gas is heated by passing through the annular arc path between the electrodes within the torch. The heated gas is then applied to the coal seam to facilitate coal conversion.
By controlling the movement of the plasma torch, more control and predictable tunneling is possible. Still, the Camacho method shares a number of other disadvantages with the previously discussed electro-thermal gasification method disclosed in the 1951 "Chemical Engineering" article.
Both processes are vulnerable to uncontrolled fires within the coal seam. These fires result from excessive heating of the coal by the applied electric potential combined with available or supplied air or oxygen. Another significant disadvantage of these electro-thermal gasification methods relates to their detrimental impact upon the natural environment. Excess contaminated water from the mined areas often seeps back into and pollutes the ground water.
In addition, these techniques lead to undesirable leakage of volatile gases that are left trapped within the coal seam. Not only do these present a subsequent explosion hazard, but they are indicative of product extraction losses and the low recovery efficiency possible with these methods. A need is therefore identified for an improved, more efficient method for the in situ conversion of coal.